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Biology A level - Ecosystems Cheat Sheet by

This is based on the OCR A Gateway spec for A level biology, Chapter 23 from module 6. Specification reference: 6.3.1

Key words for topic

Ecological / enviro­nmental area inhabited by particular species.
Group of organisms of same species in area.
Eco. unit composed of different popula­tions.
Role of an organism in an ecosystem (compe­tition occurs when 2 organisms share a niche).
All affected by biotic + abiotic factors.

Biomass transfer

Efficiency at producer level
Photos­ynt­hesis ≠ 100% of sunlight.
Net production = gross prod. - respir­atory losses
Efficiency at consumer level
Not all energy consumed (not all eaten or digested, metabolic heat...).
Ecological efficiency = (energy or biomass available after transfer / energy or biomass available before transfer) x 100


Feed on detritus (dead organic material).
Secrete enzymes in dead material to digest it.
Recycling nitrogen
Nitrogen necessary for bio molecules, e.g. nucleic acids, amino acids...
Nitrogen fixation - Nitrogen cannot be used 'raw', so needs to be bonded with other molecules (by Rhizobium and Azotob­acter).
Nitrif­ication - Ammonium compounds converted into nitrog­en-­con­taining compounds.
Requires oxygen.
Carried out by Nitros­omonas and Nitrob­acter.
Denitr­ifi­cation - No oxygen --> denitr­ifying bacteria convert nitrates into N gas.
Ammoni­fic­ation - Nitrogen into ammonium compounds.

Nitrogen cycle

Carbon cycle


Succession key terms

Type of living organisms changes over time (in plants, from annual plants to hardwood trees).
Communities become increa­singly complex, biodiv­ersity increases.
Primary succession - Plants colonise barren land for the first time.
Secondary succession - Soil already present, plants grow for first time.
Seral stages / seres
Step in succes­sion.
Defected succession
Succession halted before climax community.
If caused by human activity, then = plagio­climax.

Plant succession

Plant succesion

Pioneer community
Seeds, spores carried by wind, excrem­ent...
Produce many seeds, germinates quickly, fix N2 from atmosp­here.
Interm­ediate community
Erosion and decomp. of pioneer species form humus. Can support new species (secondary coloni­sers).
Tertiary colonisers - When conditions improve. Live with little water.
Climax community
Stable, little change over time.

Distri­bution v. Abundance

Where organisms are found in an ecosystem.
Line or belt transect normally used.
Number of indivi­duals of a species found in an area.
Animals - Captur­e-r­eca­pture.
Plants - Indivi­duals per area.

Simpson's index of diversity

D = 1-∑(n/N)2
D = diversity index
N = total number of organisms in ecosystem
n = number of indivi­duals in each species
Value between 0-1. Higher value = higher biodiv­ersity.


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